A group communications technology is a communications technology that has a scheduling capability, and may provide a unidirectional call capability, to allow user equipment (UE) to communicate with one or more other users. As mobile services become increasingly rich, a text, an image, a video, and the like become content of group communication. A conventional group communications technology can no longer satisfy a communication requirement. Currently, development of a Long Term Evolution (LTE) technology makes implementation of rich group services on a public LTE network possible. Group communication in the LTE technology may perform information transmission in a unicast manner and a multicast manner. In the multicast transmission manner of the group communication, information transmission is performed by using an existing multimedia broadcast/multicast service (MBMS) technology.
In the prior art, if UE 1 needs to send data to UE 2, UE 1 sends the data to a group communication service application server (GCS AS) by using a unicast bearer. The GCS AS determines to send the data to UE 2 in a multicast manner or a unicast manner. Specifically, the GCS AS transmits data based on the Real-Time Transport Control Protocol (RTP), that is, transmits, in a form of multiple RTP packets, the data sent by UE 1 to UE 2. Currently, a delay from a time at which the GCS AS starts sending the first RTP packet to a time at which UE 2 receives, by means of unicast, the first RTP packet sent by the GCS AS is 40 ms, that is, a transmission delay of unicast transmission is 40 ms. A delay from a time at which the GCS AS starts sending the first RTP packet to a time at which UE 2 receives, by means of unicast, the first RTP packet sent by the GCS AS is 160 ms, that is, a transmission delay of multicast transmission is 160 ms.
In a moving process of UE 2, a transmission manner in which the GCS AS sends data to UE 2 is switched, that is, when UE 2 moves from a non-MBMS coverage area to an MBMS coverage area, the transmission manner is switched from unicast to multicast; when UE 2 moves from the MBMS coverage area to the non-MBMS coverage area, the transmission manner is switched from multicast to unicast.
As shown in FIG. 1, UE 2 moves from the non-MBMS coverage area to the MBMS coverage area, and UE 2 is switched from unicast to multicast. It is assumed that switching occurs at a moment to. In this case, seven RTP packets have been transmitted on a unicast path, while the first RTP packet has just been transmitted on a multicast path. If in this case, UE 2 is switched from unicast to multicast, UE 2 receives RIP packets on the multicast path according to a sequence of sending the RTP packets on the multicast path. UE 2 repeatedly receives, on the multicast path, RTP packets whose sequence numbers are 2 to 7 and that have been received on the unicast path (for example, UE 2 may discard these repeatedly received packets, because these packets have been received on the unicast path before switching). Therefore, in a processing of switching from unicast to multicast, UE 2 repeatedly receives some RTP packets.
As shown in FIG. 1, UE 2 moves from the MBMS coverage area to the non-MBMS coverage area, and UE 2 is switched from multicast to unicast. It is assumed that switching occurs at the moment t0. In this case, only one RTP packet is transmitted on the multicast path, while seven RTP packets have been transmitted on the unicast path. If in this case, UE 2 is switched from multicast to unicast, UE 2 receives RTP packets on the unicast path according to a sequence of sending the RTP packets on the unicast path. Therefore, for the packets whose sequence numbers are 2 to 7, UE 2 does not receive these packets after being switched to unicast, and can only start receiving, according to a unicast data sending sequence, packets from a packet whose sequence number is 8. Therefore, in a process of being switched from multicast to unicast, UE 2 loses some RTP packets.
Therefore, in the current group communications technology, in a process of switching the UE from unicast to multicast, because the UE repeatedly receives packets or loses some packets, continuous reception of a communication service may be affected. Therefore, how to maintain continuity of the communication service is a problem that currently needs an urgent solution.